2008-05-06

Impact of warming on insects and other ectotherms

There has been quite a lot of press coverage of a new paper in PNAS (doi:10.1073/pnas.0709472105) by Deutsch et al. about how global warming might affect land-based invertebrates. The paper is very short, and easy to understand, so I recommend it. It's actually a simple idea, based on empirically derived "fitness curves" for different organisms. As I read it, the story comes down to the fact that tropical temperatures don't vary much during the course of the annual cycle, while at higher latitudes it does. This has affected the organisms that live in these different climate regimes; tropical organisms have come to "expect" a small temperature variance, and don't do well when the temperature changes more than normal. Organisms from places with distinct seasons are more amenable to temperature swings. This has been derived empirically as these fitness curves, which are broad for extra-tropical organisms and narrow for tropical ones. An interesting characteristic is that the maximum fitness level comes at a temperature optimum, followed by a precipitous decline. So Deutsch and co-authors followed up on this, defining a "warming tolerance" and a "thermal safety margin," which just measure how close an organism lives to its maximum temperature tolerance and to it's optimal temperature, respectively. Then they apply a warming scenario to see what happens, and it turns out that tropical insects (the data they used), get pushed really hard compared with midlatitude insects. This is somewhat surprising, since the tropics don't warm as much as higher latitudes, but because the tropical organisms live so close to their maximum temperature already, they are heavily stressed by the more moderate warming. They extrapolate to a global scale of impact on insects, and then three extra classes (lizards, frogs, turtles). This part of the paper does not seem surprising after the initial analysis, though the extension to lizards, frogs, and turtles helps deliver the message.

We often hear about the impact of climate change on biology, ecosystems, etc., but we don't often see such a concise and simple, yet far-reaching and quantitative analysis of "impact." This paper stands out to me because of these traits, and gives an excellent example to use when discussing the importance of climate change beyond temperature, precipitation, etc.

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